Thank you for reaching out on Italiapass!
Travelling from Siena to Sorrento is quite the journey! You can book most of it on Italiarail.com35 with the exception of Naples - Sorrento.
Trains to Sorrento are provided by a private trainline called the Circumvesuviana line. Tickets for these trains can only be purchased in person at the train station in Naples. Therefore, you should look to book a trip from Siena - Naples, then purchase tickets to Sorrento in person on the day of travel. The quickest route from Siena - Naples involves 1 change in Florence with a total journey time of about 5 hours on a high speed train.
If you have any questions or concerns about bookings, or anything else, feel free to reach out to us at 1 877 375 7245 or firstname.lastname@example.org